PCI Express (Peripheral Component Interconnect Express), or PCIe, is a high-speed serial computer expansion bus standard. A computer expansion bus is a computer bus that transfers information between the main hardware components of a computer (including the central processing unit and memory) and peripheral devices. A computer expansion bus includes a collection of conductors (e.g., wires or signal traces) and protocols that allow for the expansion of a computer to include peripheral devices.
PCI Express devices communicate via a connection called a link. A link is a point-to-point communication channel between the ports of two PCI Express devices allowing both of the devices to transmit and receive signals. At the physical level, a link is composed of one or more lanes. Each of the lanes in a PCIe link has two differential signaling pairs of conductors, with one pair for receiving data and the other pair for transmitting data. Thus, each lane is composed of four conductors. Each differential signaling pair of conductors in each lane transmits a differential signal in serial from one device to another device. A physical PCI Express link may contain 1, 2, 4, 8, 12, 16, or 32 lanes. Each lane in a link transports data packets in eight-bit byte format simultaneously in both directions between the endpoint devices.
PCI Express is a layered protocol that includes a transaction layer, a data link layer, and a physical layer. The physical layer includes a physical coding sublayer (PCS) and a physical media attachment (PMA) layer. The physical media attachment (PMA) layer includes a serializer/deserializer and other analog circuitry. The physical coding sublayer (PCS) performs encoding and decoding of the data as well as other functions. PCI Express also includes a media access control (MAC) sublayer, which may be part of the data link layer.